Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit with a fine pattern on e.g. a silicon substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices is being accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (193 nm) is employed, about 45 nm is presumed to be the limit. Under the circumstances, as an exposure technique for the next and subsequent generation employing an exposure wavelength shorter than 45 nm, EUV lithography is expected to be prospective, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser. In this specification, EUV light is meant for light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically for light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
EUV light is likely to be absorbed by all kinds of substances, and the refractive index of substances at such a wavelength is close to 1, whereby it is not possible to use a conventional refractive optical system like photolithography employing visible light or ultraviolet light. Therefore, in EUV lithography, a reflective optical system, i.e. a reflective photomask and mirror, is employed.
A mirror to be used for EUV lithography has a structure wherein a reflective layer for reflecting EUV light is formed on a substrate made of e.g. glass. As the reflective layer, a multilayer reflective film having high refractive index layers and low refractive index layers alternately laminated plural times is usually used, since a high EUV reflectivity can be thereby accomplished. Therefore, as a mirror to be used for EUV lithography, such a multilayer mirror having a multilayer reflective film formed on a substrate is usually employed (Patent Document 1).
In such a multilayer mirror, a protective layer (a protective capping layer) is formed on such a multilayer reflective film for the purpose of protecting the multilayer reflective film from chemical or physical erosion.
The multilayer mirror disclosed in Patent Document 1 is provided with a protective capping layer made of a material selected from ruthenium (Ru) and rhodium (Rh), and their compounds and alloys.